The present invention relates to polyurethane systems based on polyisocyanates and at least two polyesters having different melting points and to a process for bonding substrates using the polyurethane systems according to the invention.
Hotmelt adhesives are known. Their advantages are that, applied as hot melts, they solidify quickly on cooling and hence build up strength. Their disadvantages are that, due to the high melting temperatures, heat sensitive substrates are difficult to bond and the bonds soon lose strength with increasing temperature due to the thermoplastic character of the polymers.
An elegant means of building up the properties of hotmelts, namely strength on cooling, while at the same time facilitating application at low temperatures to form bonds of high heat resistance are the reactive hotmelt systems based on isocyanate-containing prepolymers of diisocyanates and polyester diols with melting ranges above 40.degree. C. which are described in German patent 878,827 and Auslegeschrift 2,609,266. Due to their low molecular weight, the products are liquid and can be applied at temperatures just above the melting range of the polyesters and, through recrystallization, develop increased initial strengths and ultimate strengths by reaction of the free isocyanate groups with, for example, atmospheric moisture to form linear polyurethane polyureas of high molecular weight.
Since systems such as these, despite their low molecular weight, are intended to build up a certain strength immediately after application of the melt, crystalline polyesters are absolutely essential to the composition of the hotmelts. Particularly suitable crystalline polyesters are the polyesters based on aliphatic dicarboxylic acids containing an even number of carbon atoms, such as for example adipic acid, sebacic acid or dodecanedioic acid, and diols, such as for example butane or hexane diol. The melting points and crystallization rate of the polyesters and, hence, of the hotmelts increase with increasing chain length of the dicarboxylic acids and the diols.
However, it has been found that systems based on very rapidly crystallizing polyesters, for example polyesters based on dodecanedioic acid and hexane-1,6-diol, lead to very brittle products, so that there are limits to a desirable increase in initial strength by rapid crystallization. There has been no shortage of attempts to overcome these disadvantages. Thus, according to European patent 107,097 for example, high molecular weight polymers are added to the isocyanate-containing prepolymers to increase initial strength. However, products such as these can only be compounded with considerable effort using expensive machines, such as planetary mixers or extruders, because the viscosity of the systems increases to a considerable extent. In many cases, compounding in this way also has an adverse effect on stability in storage.
The problem addressed by the present invention was to provide polyurethane systems characterized by rapid crystallization, high flexibility after crystallization and good curing.